Iran-backed militias are preventing civilians and opposition fighters from leaving the besieged districts of east Aleppo as Russia struggles to convince the Assad government and allied militants to abide by a ceasefire agreement.

Shelling of the besieged districts resumed on Wednesday morning despite the agreement brokered by Turkish intelligence and the Russian military on Tuesday that would have offered a respite to tens of thousands of trapped civilians.

It was unclear on Wednesday when residents would be allowed to leave east Aleppo and whether the deal would hold. Turkey’s state-run Anadolu agency quoted the head of the Turkish Red Crescent as saying nearly 1,000 people from east Aleppo were being held at an Iranian militia checkpoint.

Rebels inside east Aleppo said they would support the agreement but Iranian-backed militias on the ground, which led the assault into east Aleppo, were blocking it because the deal was reached without Assad or Iran’s involvement.

“The sectarian militias want to resume the massacre in Aleppo and the world has to act to prevent this sectarian slaughter led by Iran,” said Bassam Mustafa, a member of the political council of Noureddine Zinki, one of the main rebel groups in east Aleppo. “The opposition will continue to abide by the agreement.”

Yasser al-Youssef, a spokesman for the group, said Russia was attempting to convince the Assad government to accept the ceasefire.The Turkish foreign minister, Mevlüt Çavuşoğlu, said discussions were ongoing with Russia and Iran to continue the planned evacuations.

Russia and Turkey negotiated the agreement apparently without the Assad regime’s knowledge. The Syrian military initially said it had no knowledge of the deal before backtracking and saying the evacuations would begin on Wednesday at 5am Aleppo time. The evacuation of rebel-held eastern Aleppo has however yet to begin.

The confusion over the deal, which imposed a ceasefire at 6pm local time on Tuesday, highlighted the splits and competing interests of Assad’s supporters, which include Russia, Iran and Iran-backed militias on the ground, who were on Tuesday implicated by the United Nations in execution-style shootings of civilians in opposition areas.

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Residents said intense shelling had resumed on Wednesday in their shrinking, besieged enclave, where they had endured what the UN described asabrutal “meltdown of humanity” as forces loyal to Assad rampaged through newly reclaimed districts.

Russia said the renewed shelling was a response to rebel attacks.

The French foreign minister, Jean-Marc Ayrault, said confusion surrounding the evacuation showed it was imperative to have UN observers to manage the process. “France wants the presence of UN observers on the ground and humanitarian organisations like the Red Cross must intervene,” he told France 2 television.

There had been conflicting accounts of the expected start times for the evacuation. A military official in the pro-Assad alliance had said the evacuation was due to start at 5am (3am GMT), while opposition officials said they had been expecting a first group of wounded people to leave earlier.

However, none had left by dawn, according to a Reuters witness at the agreed point of departure. Twenty buses were waiting with their engines running but showed no sign of moving into Aleppo’s eastern districts.

“There is certainly a delay,” said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, a war monitor. The pro-opposition Orient TV cited its correspondent as saying the plan might be delayed until Thursday.

The United Nations said on Wednesday it was not involved in plans to evacuate fighters and civilians, but it was ready to help.

“[The UN] stands ready to facilitate the voluntary and safe evacuation of injured, sick and vulnerable civilians from the besieged part of the city,” the UN Office for the Coordination of Humanitarian Affairs said in a statement.

A senior Turkish official told the Guardian that Ankara and Moscow would act as guarantors of the ceasefire agreement, which would allow “civilians and moderate rebels with light weapons” to leave Aleppo for Idlib province. “Once they reach Idlib, they will be free to relocate,” the official added.

Rebel officials from Nour al-din al-Zenki and Ahrar al-Sham, two powerful opposition groups with a presence in the city, confirmed the deal to evacuate, but there was confusion over exactly where the opposition would have to go, with rebels saying they would be transported to the western Aleppo countryside.

Weeks of immense suffering and violence in east Aleppo since the Syrian regime and allies began a final push into territory that had been in rebel hands since 2012 have left residents in total despair and increasingly angry at the international community for abandoning them to their fate.

The US ambassador to the United Nations, Samantha Power, told the security council that the Syrian government, along with Russia and Iran, bore responsibility for the deaths of civilians in Aleppo. She accused the three states of putting a “noose” around civilians in the city, asking: “Are you incapable of shame? … Is there no execution of a child that gets under your skin? Is there literally nothing that shames you?”

The United Nations said it had received reliable reports from multiple sources that pro-Assad forces, including the Iraqi Shia militia Harakat al-Nujaba, had carried out summary killings of at least 82 civilians, including 11 women and 13 children, in four different neighbourhoods of east Aleppo that had fallen under government control.

Iranian leaders were self-congratulatory on Wednesday at the role it had played in the assault. The chief military adviser to the supreme leader, Ayatollah Ali Khamenei, said the city was “liberated thanks to a coalition between Iran, Syria, Russia and Lebanon’s Hezbollah.”

Reports of killings by the advancing forces raised grave concerns over the fate of tens of thousands of civilians, doctors and activists who have remained in the shrinking rebel enclaves, and who faced death if they stayed there or being tortured and killed in regime-held areas if they fled over government lines.

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Reuters reported that residents in eastern Aleppo were burning personal possessions as they prepared to leave, fearing looting by the Syrian army and Iranian-backed militias.

“I really, really hope this deal will materialise because the suffering of civilians on both sides has been immense,” said Pawel Krzysiek, head of media at the International Committee of the Red Cross, who was in Aleppo. “There is a human tragedy happening all over the city. People who lost everything are suffering here enormously.”

The evacuation of east Aleppo means the opposition will cede the entire city, Syria’s former commercial capital, to the Assad regime, surrendering the last major urban stronghold where it maintained an active presence.